Question

Flatworms were one of the first to develop a , which allowed them to detect light, food, and mates.

Answer 1

Head

Flatworms were the one to first develop a head, which displayed a cephalic presentation. They present the cephalon which in definition were the first organisms that has had a head, or a head like structure that many organisms as of now has. 

Answer 2

the answer to your question  is head